HEAT TREATMENTS OF A MICROWAVE SINTERED SILICON NITRIDE

Abstract

The sintering of silicon nitride ceramics follows the rationale that the final mechanical properties ofthe material are directly linked to the thermo-mechanical history ofthe liquid phase of sintering. Post sinter heat treatments performed on the as sintered material have the potential to alter the character of the secondary phase, and therefore modify the mechanical properties ofthe material. In the present study a mixed a/p SiAlON ceramic was manufactured by microwave sintering, then post sinter heat treated under various conditions. Analysis of the microstructure as a function ofthe heat treatment conditions oftemperature, pressure, and time was performed using quantitative x-ray analysis, and scanning and transmission electron microscopy. The properties of hardness and fracture toughness were also classified as a function of the said processing conditions. Microstructural mechanisms responsible for the changes in mechanical properties were identified and their implications explored. Increasing temperature and time were found to degrade the hardness ofthe material due to an increase in the oc —> p phase transformation. Pressure was found to have no effect on the hardness The p SiAlON grain size increased with increasing temperature. The increase in grain size followed grain coarsening behaviour, and was found to effect a small increase iii in fracture toughness when time was increased from 30 to 120 minutes; longer times had no similar effect. The grain size was predicted to have increased significantly after long times, and remain unchanged as a function ofpressure. The secondary crystalline phase was discovered to be the most crucial microstructural parameter responsible for the fracture toughness. The chemistry and high degree of crystallinity ofthe secondary phase resulted in an intergranular fracture mode, which led to toughening by crack deflection, fiber pullout, and elastic bridging. The amount of secondary phase was found to decrease with increasing temperature and pressure, thereby reducing the amount ofpossible toughening, and causing a decrease in the fracture toughness. The discoveries made in this study have resulted in the development of process optimization guidelines for the post sinter heat treatment of the current material.